The present invention relates to a propeller windmill for a small-sized power generator, and more particularly to a small-sized propeller windmill for small-scaled power generation.
In general, a windmill used in wind power generation enables the acquisition of electric power by making use of strong natural energy. On the of hand, it is necessary to for the windmill to take a measure such that a structure or a device which makes the windmill withstand a strong wind or let a strong wind go apart therefrom in a natural environment is added to the windmill. Accordingly, the following various countermeasures against a strong wind have been applied to conventionally known propeller windmills.
For example, patent literature JP-A-2003-56448 discloses the invention where pitch angles of a plurality of blades mounted on a propeller windmill are changed and controlled independently by a mechanical mechanism housed in a hub of the propeller windmill.
On the other hand, since a small-sized propeller is arranged near a ground surface, the pitch-angle changing and controlling mechanism having the complicated constitution described above cannot be incorporated into the inside of the small-sized propeller windmill. Accordingly, a countermeasure against a strong wind or a countermeasure against wind shear is taken by increasing strength of the blades of the windmill and strength of a blade mounting portion.
However, the above-mentioned measures against a strong wind have a drawback that the starting performance and the power generation efficiency of the windmill deteriorate when a wind speed is low.
In view of the above, there has been developed a small-sized propeller windmill which can efficiently generate power ranging from when a wind speed is low to when a strong wind blows, and can also prevent the breaking of the windmill even when a strong wind blows, that is, a PROVEN windmill (see patent literature JP-B-25-3964, patent literature JP-B-29-8608, patent literature JP-A-9-79127, patent literature JP-A-9-79127, and patent literature Japanese Patent No. 3435540).
Further, in a downwind-type small-sized windmill, that is, in a windmill where a generator and windmill blades are arranged on a downstream of a vertical support strut which constitutes a yawing axis, a weathercock stabilizing means which generates a yawing rotational moment is adopted so that the blades are always displaced to a downwind position.
It is found, however, that the above-mentioned structure cannot ensure the sufficient weathercock stability in case of the downwind-type small-sized windmill.
It has been considered in general that, in the downwind-type windmill, provided that a rotational plane of a windmill blade is arranged more on leeward side than a vertical support strut which constitutes a rotational axis in the yaw direction, a rotational front plane of the windmill blade is always displaced to a leeward side due to the positional relationship between the wind-pressure center positioned at the approximately center of the blade rotational plane and the yawing rotational axis. However, according to an experiment which was actually carried out, it is found out that, in a small-sized windmill which has a diameter of rotation of an approximately 50 cm, even when a wind speed is 5 m or below, it is not always the case that a rotational front plane of a windmill blade is displaced to a leeward side and the rotational front plane returns to an original upwind position where the rotational front plane faces a wind.
Such a phenomenon is logically explained in conjunction with FIG. 12. Symbol 100 indicates windmill blades, That is, FIG. 12 is a view for explaining a phenomenon where the downwind-type windmill is stabilized at an upwind position. In FIG. 12, it is estimated that, when a blade rotational plane 120 does not face a wind and has a slight yawing angle about a yawing axis Z1, the blade rotational plane 120 functions as a disk wing so that a vortex V is venerated on a back side of a blade wing end region 110 on a windward side of the blade rotational plane 120 whereby a negative pressure is increased and the negative pressure imparts a moment M1 which returns the blade rotational plane 120 to the direction where the blade rotational plane 120 faces the wind. In the drawing, symbol F1 indicates the flow of air.
Further, in a conventional micro wind power generator, a load applied to windmill blades when a strong wind blows is large and hence, it is necessary to impart the strong structure to the blades per se. It is also necessary to ensure the strong connection between the blades and an input shaft of a generator. Accordingly, the combination of the configuration and structure of the windmill blades with a power generation capacity of the power generator is fixed and limited and hence, it is impossible for the conventional micro wind power generator to change the configuration and the structure of the windmill blade corresponding to an output load.
In general, in designing a windmill, the windmill is designed so as to absorb wind power at maximum. Accordingly, windmill blades are designed such that maximum efficiency is acquired at times where a required torque is outputted at respective different rotational speeds. Accordingly, for generating the maximum efficiency, in a usual micro wind power generator, power generation performance on a power generator side is synchronized such that a predetermined resistance torque is inputted to the windmill for every acquired rotational speed.
On the other hand, on a power generator side, there exists a circumstance where, when a value of an electric current which flows into a load is changed, a generation torque differs even at the same rotational speed whereby a resistance torque cannot be determined flexibly corresponding to an output load.